About three weeks ago our 10 year old blower motor in the furnace (which also blows for the AC) died (the bearing seized). HVAC company comes out, removes the squirrel cage and motor, puts a new motor (with a different motor brand/model) on the same squirrel cage, and re-installs it. It's fine for a day or two, but now there's a noise when the fan kicks on. Best way I can describe it is that it sounds like a loud electrical buzzing (almost like a buzzer on a dryer, even). The noise disappears after about 3 seconds, but occasionally if you stand near one of the return registers, the fan sounds makes a rhythmic tapping -very- much like a diesel engine on a truck (at idle), though it sounds very distant. The fan is not belt driven and the motor is attached to the squirrel cage using rubber washers. After a run cycle, there is no excess heat and we use basic 90-day MERV 8 pleated filters (nothing fancy). The AC coil is a little dirty, but the fan is clean. Is this noise just the noise this particular motor makes or should I be concerned?
Some motors have a start up noise, and is normal for that type of motor. My table saw does this. Starts out a loud 60 Hz hum which quickly increases in frequency as the motor spins up, then gets inaudible or washed out by rushing air when full speed is reached. Motors that do this are not really appropriate for residential blowers.
It also possible it's not a normal noise and there is a defect. Very hard to differentiate on the internet. The rhythmic noise sounds like the squirrel cage is slightly hitting something. Perhaps it is misaligned or out of balance. Misalignment or balance issues could also cause the motor to make unusual noises. Something is not right and warrants further investigation. Could very well be minor, or a warning of something that will get worse.
Even if the start up noise were normal, I would be unhappy that a formerly quiet motor was replaced with a noisy one.